1. Field of the Invention
This invention relates to a charging member, and a process cartridge and an electrophotographic apparatus both of which have the charging member.
2. Related Background Art
Image forming apparatus that employ an electrophotographic system, called electrophotographic apparatus, commonly have an electrophotographic photosensitive member, a charging means, an exposure means, a developing means and a transfer means.
In the charging means, a system is primarily employed in which a voltage (a DC voltage only or a voltage created by superimposing an AC voltage on a DC voltage) is applied to a charging member disposed in contact with, or in proximity to, the surface of an electrophotographic photosensitive member, to charge the surface of the electrophotographic photosensitive member electrostatically.
In the case where the voltage created by superimposing an AC voltage on a DC voltage is employed as the voltage applied to the charging member, an AC power source is necessary which requires a large-sized electrophotographic apparatus or brings about an increase in cost, thus a larger power consumption may result, and higher levels of ozone may be produced because of the use of alternating current which will cause a lowering of the durability (running performance) of the charging member or electrophotographic photosensitive member. Accordingly, it is preferable that the voltage to be applied to the charging member is only a DC voltage.
In addition, from the viewpoint of stabilizing charge, reducing ozone generation or achieving low cost, a contact type charging system is preferably used.
In the case of such a contact type charging member, the charging member is kept in contact with the electrophotographic photosensitive member by the pressing force of springs or the like, and is rotated following the rotation of the latter. In many cases, the force by which the charging member is kept in contact with a member to be charged is constant.
After products such as electrophotographic apparatus and what is called the process cartridge in which the main components of the electrophotographic apparatus are integrally held, have been manufactured and before users use them for the first time, there is a possibility that the charging member and the member to be charged will be left standing over a long period of time of from a few weeks up to a few years while the former is kept in contact with the latter by the pressing force of springs or the like. There is also a possibility that, where a user does not use the electrophotographic apparatus, it will be of course left standing for a long term while they are kept in contact with each other. When thus left standing over a long period of time, the charging member is kept in a deformed state at the contact portion between the charging member and the member to be charged, and is not able to return to the original shape thus causing deformation or settling due to compression set, i.e. what is called C-set deformation (hereinafter “C-set”).
In recent years, the electrophotographic apparatus is required to achieve much higher process speed, image quality and running performance. Studies made by the present inventors have revealed that such requirements cause the contact zone (C-set areas) to affect images more conspicuously. The C-set areas may appear as horizontal black lines and/or horizontal white lines (C-set images) in the longitudinal direction when, e.g., halftone images are reproduced. This is known to be due to non-uniform charging of the charging member. It has also come to light that the above C-set images tend to occur especially where the voltage applied to the charging member is only a DC voltage.
As a countermeasure against such a problem of C-set, for example, Japanese Patent Application Laid-open No. H10-48913 (Patent Document 1) discloses that an elastic layer is incorporated with a copolymer of ethylene and propylene which contains as a copolymer component a diene component having an iodine value of from 23 to 32, whereby the C-set can be remedied. However, according to studies made by the present inventors, they have come to know that the technique disclosed in Patent Document 1 can not sufficiently remedy the C-set under severer conditions, e.g., under such circumstances that the voltage applied to the charging member is only a DC voltage.